Catalytic reforming, or hydroforming, is a well-established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation/dehydrogenation (hydrogen transfer) component, or components, composited with a porous, inorganic oxide support, notably alumina. Platinum metal catalysts, or a catalyst which contains platinum to which one or more additional metal promoters have been added to form polymetallic catalysts, are conventionally employed in conducting reforming operations. In a reforming operation, one or a series of reactors constitute the reforming unit which provides a series of reaction zones. Typically, a series of reactors is employed, e.g., three or four reactors, these constituting the heart of the reforming unit. Each reforming reactor is generally provided with a fixed bed, or beds, of the catalyst, each receives down-flow feed, and each is provided with a preheater or interstage heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, is concurrently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a C.sub.5 + liquid fraction which is recovered, and a vaporous effluent. The vaporous effluent is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated and recycled to the process.
Reforming is defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. The recycled hydrogen suppresses, but cannot prevent the build up of coke.
The sum-total of the reforming reactions in most commercial operations occurs as a continuum between the first and last reaction zone of the series, i.e., as the feed enters and passes over the first fixed catalyst bed of the first reactor and exits from the last fixed catalyst bed of the last reactor of the series. During an on-oil run, the activity of the catalyst gradually declines due to the build-up of coke on the catalyst, and hence during operation, the temperature of the process is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processing of this type the catalyst must necessarily be periodically regenerated by burning off the coke in the presence of an oxygen-containing gas at controlled conditions. Catalyst reactivation is then completed in a sequence of steps wherein the metal hydrogenation/dehydrogenation components are atomically redispersed.
In the reforming operation, the reactions which predominate between the several reactors differ principally by the nature of the feed, and the temperature employed within the individual reactors. In the initial reaction zone, or first reactor, which is maintained at a relatively low temperature, it is believed that the primary reaction involves the dehydrogenation of naphthenes to produce aromatics. The isomerization of naphthenes, notably C.sub.5 and C.sub.6 naphthenes, also occurs to a considerable extent. Most of the other reforming reactions also occur, but only to a lesser, or smaller extent. There is relatively little hydrocracking, and very little olefin or paraffin dehydrocyclization occurs in the first reactor. Within the intermediate reactor zone(s), or reactor(s) of the first stage, the temperature is maintained somewhat higher than in the first reforming zone, or first reactor of the series, and it is believed that the principal reaction involves the isomerization of naphthenes, normal paraffins and isoparaffins. Some dehydrogenation of naphthenes may, and usually does occur, at least within the first of the intermediate reactors. There is usually some hydrocracking, at least more than in the first reactor of the series, and there is more olefin and paraffin dehydrocyclization. The tail reactor of the first stage is generally operated at a somewhat higher temperature than the second reactor of the series. It is believed that the naphthene and paraffin isomerization reactions continue as the primary reaction in this reactor, but there is very little naphthene dehydrogenation. There is a further increase in paraffin dehydrocyclization, and more hydrocracking. In the second reforming stage which is operated at the highest temperatures, it is believed that paraffin dehydrocyclization, particularly the dehydrocyclization of the short chain, notably C.sub.6 and C.sub.7 paraffins, is the primary reaction. The isomerization reactions continue, and there is more hydrocracking in the reforming zones of this stage than in any one of the reforming zones of the first stage. Unfortunately, dealkylation and hydrocracking reactions occur to some extent throughout all of the reforming zones of a unit.
It is, accordingly, a primary objective of this invention to provide an improved process for the manufacture of relatively high octane gasoline at high severity reforming conditions.
In particular, it is an object to provide a high severity reforming process for the production of relatively high octane gasoline by utilizing two reforming stages. The first reforming stage is comprised of one or more fixed-bed reforming zones containing a highly active supported platinum catalyst, promoted with iridium; and additionally, small, critical amounts of tin. The amount of tin is sufficient for this platinum-iridium-tin catalyst to produce an intermediate range octane (80-95 RON) gasoline from low octane naphtha, while suppressing dealkylation in the lead reactors as well as moderating hydrocracking reactions which occur in downstream reforming zones.
A further object is to provide a process, as characterized, but further utilizing a platinum-tin catalyst in one or more of the moving-bed reforming zones of the second reforming stage. This fixed-bed/moving-bed process will produce greater yields of C.sub.5 + reformate and hydrogen than otherwise similar two-stage processes except that they do not contain tin modified platinum-iridium catalysts in the reforming zones of the first stage. Also, cycle length of the first reforming stage will be increased by use of the platinum-iridium-tin catalyst.